Six groups of young giant breed dogs, Great Danes, were raised on food differing in content of either protein (13%, 21% or 30%) or mineral (1.1% Ca and 0.9% P, 3.3% Ca and 0.9% P or 3.3% Ca and 3.0% P), but not in energy. Plasma GH and IGF-I concentrations were determined during the study, i.e. from the 6th to the 26th week for the protein groups and to 17 weeks for the mineral groups. When compared with miniature poodles, the Great Danes had significantly higher levels of GH and IGF, the former decreasing with age. Low protein intake as well as high Ca intake (but not high Ca and high P intake) were associated with decreased IGF-I but not with different GH levels, when compared with the control dogs. Low protein intake may result in decreased hepatic IGF-I synthesis. Hypoparathyroid-induced decreased autocrine and paracrine IGF-I formation in the dogs with high Ca intake and slower growth rate is discussed.